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Chapter 8

A Scientific Perspective on Composting

John K. Lekasi, Keziah W. Ndung’u and Mary N. Kifuko

From a scientific perspective, composting is the partial decomposition and stabilization of heterogeneous organic substances by a mixed microbial population under optimum conditions of moisture, temperature and aeration. Compost is composed of fairly homogeneous, stable organic matter with high molecular weight and is free of pathogens and weeds seeds. In nature, dead organic materials undergo several processes of microbial transformation according to substrate composition and physical environment. Composting requires that the process be mostly aerobic so that the organic matter is partially mineralized and humified. In order to produce suitable compost for agriculture, the composting process must be controlled, particularly through the choices of substrate, moisture content and aeration. Many of the different composting systems that have been developed have utilized this basic principle in their design (Table 1).

Many agricultural wastes contain sufficient organic material for composting. These include solid urban waste, food factory waste and other industrial by-products, sewage sludge, agricultural residues and domestic waste. Some of these wastes require careful separation of organic matter from inert materials such as glass, plastic and metals. The organic fraction of the waste is quite heterogeneous and if introduced directly into the soil, it will not perform in as predictable a manner. Composting radically transforms various organic substances, it mineralizes the readily assimilable materials and humifies them into more complex compounds. Stated more simply, composting is an elegant, but rather complex process that blends different ingredients into a uniform useful product.

Stages of Composting

The composting process is characterized by a period of rapid decomposition and temperature accumulation followed by cooler, slower decay of the remaining organic substrates. The rate of decomposition can be increased by stacking the materials in a pile to a height of 1.0 to 1.5 m, however, taller  stacks  must  be more frequently turned to facilitate rapid decomposition and prevent the formation of unwanted anaerobic by-products.

The temperature in the compost pile will rise due to enhanced microbial (fauna and flora) heat production resulting from heterotrophic oxidation and also because of the relatively slow heat transfer to the environment. Other important aspects of composting include the C/N ratio of the organic substrates, other nutrient concentrations, surface area and acidity. Microorganisms, primarily bacteria, actinomycetes and fungi, use the organic materials as a source of energy and nutrients, producing heat, gases and stabilized organic matter. Microbial populations change with temperature during the mesophilic (20-40 ⁰C) and the thermophilic stage (>40 ⁰C) then back to ambient during the curing stage (Figure 1).

Table 1. A summary of composting systems for organic wastes which are also utilized for industrial composting (adapted from Bertoldi et al., 1985).

Open Systems

• Turned pile (commonly used in Kenya)

• Static pile     

  • air suction

  • air blowing

  • alternating ventilation (blowing and suction)

  • air blowing in conjunction with temperature control

Closed Systems

• Vertical reactors 

  • continuous

  • discontinuous

• Horizontal reactors       

  • static

  • with movement of material

The mesophilic stage is a preparatory state that  initiates the decomposition process and brings the compost into temperature ranges that are suitable for thermophiles. This stage is achieved by the rapidly decomposing and readily-available compost substrate. In the mesophilic stage, temperatures rise rapidly to high levels up to 65⁰C.

The thermophilic stage is necessary to ensure stabilization and to  pasteurize the compost, eliminating many harmful organisms. This stage may last a number of days depending on how well oxygen is supplied to the pile and the quality and quantity of the substrate.

Temperatures vary in a compost pile with the outer layer having a lower temperature compared to the inner zone of high temperature (Figure 2). To ensure even decomposition and better aeration, periodic turning is necessary.  Enteric pathogens such as Salmonella spp., normally survive less than one hour once the compost enters the thermophilic stage. Other noxious organisms such as weed seeds and parasites eggs may be more durable but are often eliminated or greatly reduced during composting. Pasteurization is one reason composting is a popular waste treatment. During the stabilization stage, substrate becomes a limit to microorganisms. The compost pile temperatures fall back to mesophilic stage range and reestablishment of the mesophilic organisms occurs. During slower processing, the compost may become colonized by soil fauna, such as earthworms or beetles, and these organisms assist in curing.

Properties of compostable materials

The important physical properties of materials intended for composting are particle size and moisture content. Particle size affects oxygen movement into and within the pile, as well as microbial and enzymatic access to the substrate. Proper balance in the particle size should be maintained. If too large, the organic materials should be chopped into smaller pieces. On the other hand if too small, the organic materials should be mixed with a bulking agent (eg. wood chips or tree bark). The optimum moisture content for composting is 40 to 60%. Water interferes with oxygen accessibility, slowing the rate of composting while too little water hinders diffusion of soluble molecules and microbial activity.

The chemical characteristics of the organic residues may be considered in terms of nutrient quality and quantity. The ranges of elemental composition for some residues suitable for composting are shown in Table 1. The relative quantity of the C, N, P S and other nutrients is important, but keep in mind that mineral nutrients are largely concentrated during composting as the carbon compounds are oxidized by microorganisms. Substrate quality is also influenced by secondary compounds, such as lignin and polyphenol, that are more recalcitrant to decomposition and may restrict nitrogen availability through proteins-binding.

Table 2. Nutrient concentrations of selected dry compostable materials

Although different compostable substrates often begin with widely divergent nutrient contents, the quality and quantity of the nutrient converge as composting proceeds. Metabolic processes also affect the pH of the compost. Deamination of proteins rapidly increases the pH due to ammonia. Conversely, production of organic acids during the decomposition of carbohydrates and lipids decreases the pH. On average, pH of inputs is somewhat acidic while finished compost is near neutral.

The relative quality and quantity of the organic residues affects the rates of composting and the characteristics of the finished products. For example, when the C/N ratio of the organic matter is about 25, metabolism of the organic material may proceed rapidly with a high degree of efficiency of N assimilation into the microbial biomass. A narrower C/N ratio may lead to loss of N from compost through ammonia volatilization. Wider C/N ratios (>40) promote immobilization of available N in the compost slowing the rate of decomposition. Therefore, addition of mineral N and P in the process of fortification can enhance rapid decomposition and enrichment of low quality residues.

Assessing Compost Maturity

Compost most suitable for agriculture should be well cured and mature. The basis for efficient preparation of compost hinges upon recognizing differences in quality and adjusting application rates and timing of application accordingly. At present, as well as the traditional tools for investigating decomposition (C:N ratio, temperature, humidity) other methods are available from the most sophisticated, which may be employed only in well-equipped laboratories, to the most basic which can be adapted to the immediate needs of a smallscale composting operation.

A study was conducted in Maragua District in Kenya to identify simple methods of rapidly assessing the quality of composted cattle manure (Lekasi et al., 2003). Several discernible characteristics could be used to judge maturity and quality of these composts including texture, colour, smell and biological activity.

Table 3. Chemical characteristics of some mature compost entered into the FORMAT Compost Contest in 2002.

When compost texture was considered, coarse materials become finer over time until a fine, loamy material is produced. Changes in the colour of the compost can also tell its quality and maturity. The assumption for this parameter is that less decomposed material consists of a more heterogeneous mixture of animal feaces and other organic materials that also differing in color, resulting in a mottled appearance. As decomposition progresses, such material becomes more homogeneous, appearing as uniform dark brown or black at maturity. When composting cattle manure, sewage sludge and some industrial wastes, the smell can indicate the stage of composting. Fresh animal manure and wastes have a strong smell of ammonia and putrefaction during the early stages of decomposition. Mature compost is expected to have only a slight ‘earthy’ and inoffensive smell.

Biological activity is another useful indicator of compost maturity. The presence of macrofauna in maturing compost, particularly earthworms and grubs, serves as an indication of the stage of compost maturity because time is required for these invertebrates to re-colonize the substrate following the thermophilic stage. The fauna and flora of compost heaps changes with time, both increasing and decreasing with maturity depending on the group of organisms. For example, earthworm activity might increase to a maximum and then decline towards maturity, while other soil fauna and fungi demonstrate peak activity at other times. Grubs (beetle larvae) are often in mature compost heaps. A clear understanding of changes of these different domains in respect to the composting process and stages can, therefore, be used in combination to predict the quality and maturity of compost to a fairly accurate extent.

Maximizing Decomposition during Composting

In composting for waste management purposes, odour control and cost effectiveness are both served by maximizing the rate of decomposition. The composting “ecosystem” tends to become self-limiting by excessive accumulation of metabolically-generated heat, leading to inhibitively high temperature. The threshold to significant inhibition is approximately 60 ^oC, and inhibition increases sharply at higher temperatures. Unless controlled through deliberate venting, composting masses may reach as much as 80 ^oC, at which point the rate of decomposition becomes extremely low.  Prolonged temperature exposure literally pasteurizes the decomposing substrate, destroying many harmful organisms.

A practical means of removing heat from the composting mass is through ventilation. The main ventilation-association mechanism of heat removal is the vaporization of water. Ventilation also supplies O₂ for aerobic decomposition. During composting, the rate of heat generation varies with time. At the small-scale level, occasionally turning the heaps serves this purpose very well.  In the most advanced commercial composting, ventilation is achieved by using fans that are mechanically controlled with temperature and moisture sensors. When an initial substrate contains a large proportion of water, as with manure slurries or sewage sludge, composting also serves as a effective means of removing the excess water in addition to producing quality organic fertilizer for use in agricultural production.

Conclusion

Fortunately most of the composting science is intuitively considered by those with experience. Mixing and layering inputs results in a wide range of nutrients and substrates. Covering the compost pile reduces water loss and conserves heat. Placing the pile on a platform and periodic turning facilitates aeration. Composting is one example where the considerations of science are well covered by those who approach it as an art.

References

De Bertoldi, M, Vallini, G and Pera A. 1985. Technological aspects of composting including modelling and microbiology. In: Gasser, J.K.R. (Ed.) Composting of Agricultural and Other Wastes. Elsevier Applied Science Publishers, Essex, England. pp 27-41.

Lekasi, J. K. Tanner, J. C. Kimani, S. K. and Harris, P.J.C. 2003. Cattle manure quality in Maragua District, Central Kenya: Effect of management practices and development of simple methods of assessment. Agricultural Ecosystems and Environment 94:289-298.

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